1. Field of the Invention
The present invention relates to a base station, a control station, and a radio communication control method for controlling switching at a cell change in a mobile communication system.
2. Description of the Related Art
The common mobile communication system using cellular phones provides a communication service by dividing the entire service area into radio zones called cells. Such a system consists of a plurality of base stations covering cells, mobile stations communicating with the base stations by establishing radio channels with the base stations, and control stations managing the plurality of base stations. The control station is connected with a core network including a switching equipment and the like. In such a configuration, data transmission in a communication between a core network and a mobile station is performed through a control station and a base station. The base station has a function of temporally buffering data sent from the core network and the control station so as to relay between a wired zone between the core network, the control station and the base station, and a radio zone between the base station and the mobile station, in particular. A high-speed, large-capacity downlink high-speed packet communication such as the HSDPA (High Speed Downlink Packet Access) or 1x EV-DO is standardized and still being developed (for example, see “3GPP TR25.848 v4.0.0” and “3GPP2 C. S0024 Rev.1.0.0”). Such a high-speed packet communication method is mainly characterized in that an adaptive modulation and coding method is adopted according to a radio condition of the mobile station that is performing data reception. In the adaptive modulation and coding method, a radio transmission rate can be changed in time according to a radio condition of the mobile station. In the HSDPA, for example, throughput is improved by controlling a modulation scheme of a radio channel, a number of High Speed-Physical Downlink Shared Channel (HS-PDSCH) codes, and a coding rate according to a radio condition between a mobile station and a base station.
It is noted that the control station is called “Radio network controller” in the 3GPP system.
In a mobile communication system with the abovementioned configuration, when a transmission rate of the wired zone is higher than a transmission rate of the radio zone, data tends to accumulate in a temporary buffer of a relaying base station. When a transmission rate of the wired zone is lower than a transmission rate of the radio zone, data tends to be exhausted in a temporary buffer of a relaying base station. The temporary buffer of the relaying base station is limited. Data which overflow the buffer is discarded or the buffer of the relaying base station is exhausted and a radio resource cannot be efficiently used. Therefore, a difference between the transmission rate of the radio zone and the transmission rate of the wired zone may cause degradation of transmission efficiency lead by discarding of a data due to a buffer overflow or a lowering of an efficiency of a radio resource usage due to exhaustion of a buffer. It is needed to control to match the transmission rate of the radio zone and the transmission rate of the wired zone as much as possible to solve the two problems.
From a viewpoint of restraint of discarding of a data at a base station in a communication between a core network and a mobile station, a method for controlling the amount of inflow data from a control station to a base station based on the amount of data retained in a buffer of a base station has been proposed (for example, see JP2002-77987A). Also in a communication system such as a high-speed packet communication method in which a transmission rate in the radio zone largely fluctuates, a method for preventing degradation of transmission efficiency of a system caused by degradation of efficiency of radio resource usage due to exhaustion of a buffer and occurrence of a resending control caused by discarding of a data due to over flow of a buffer has been proposed (for example, see JP2005-57323A).
Referring to FIG. 12, a method for controlling the amount of inflow data disclosed in JP2002-77987A and JP2005-57323A will be described. Signals unnecessary for the description will be omitted.
First, when a control station is receiving data destined to a mobile station from a core network, the control station sends the data toward a base station in accordance with a transmission rate of the base station (step S601).
The base station sends the data at the radio transmission rate toward the mobile station (step S602). If the base station receives data at a transmission rate faster than the radio transmission rate, it cannot immediately send the data, thus, it temporally stores the data in a buffer of the base station (step S603).
At this moment, the amount of data stored in the buffer divided by the radio transmission rate is a “buffer retention time”, which is a time for sending all the data in the buffer. The base station monitors whether the buffer retention time exceeds a threshold or not. If the buffer retention time exceeds the threshold, the base station notifies “an inflow amount control signal” for controlling the inflow amount from a control station 50 by a sub-layer signal (step S604). The control station 50 that received the inflow amount control signal sends the data based on the amount of sent out data specified in the control signal.
To the contrary, if the radio transmission rate increases (step S606) and the amount of buffer (buffer retention time) decreases (step S607) to less than the threshold, while data are being sent (step S605), the base station notifies the control station of information for releasing sending control by a sub-layer signal (step S608). The control station that received the information for releasing the control changes the amount of sent out data based on the amount of sent out data specified in the control information. By using the method, the inflow amount to the base station can be restricted with the amount of buffer retention and overflow of a buffer at a base station can be avoided.
When a mobile station moves across cells in such a mobile communication system, switching of a base station called a cell change is done. At the cell change occasion, the control station first sends a control signal for the cell change to inform a source base station and a target base station of a timing of the cell change. At this moment, the above-mentioned controlling of the inflow amount of data from the control station to a base station has not been considered. Here, a “source base station” is defined as the base station from which the mobile station receives data before the cell change, and a “target base station” is defined as the base station from which the mobile station receives data after the cell change.
In a packet communication such as the abovementioned HSDPA, when a cell change occurs between base stations, a control station sends “a control signal of a cell change” to inform a source base station and a target base station of a timing of the cell change. It switches its destination so that downlink data flows from the control station to the target base station at the timing. There was a problem in that data sending is stopped until all the operation of the cell change finishes, i.e. continuous communication cannot be achieved. In a method for sending data to achieve continuous communication even after a control signal of the cell change is received, if data flowed from a control station to a source base station remains and the source base station cannot send all the remaining data before the cell change timing, the data cannot be correctly received by the mobile station. There was a problem in that, unless the data does not appropriately flow from the control station to the target base station, continuous communication cannot be achieved.
The invention is adapted in view of the abovementioned problems, and intends to provide a base station, a control station and a radio communication control method which can prevent breakage of data sending toward the mobile station or data loss at the cell change.